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OBJECTIVELY MEASURED PHYSICAL ACTIVITY
AND RELATED FACTORS IN MINORITY YOUTH
by
Britni Ryan Belcher
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PREVENTIVE MEDICINE—HEALTH BEHAVIOR)
May 2011
Copyright 2011 Britni Ryan Belcher

Obesity prevalence is increasing and physical activity levels are declining in US youth. The overall goal of this dissertation was to examine the effects of individual demographic, biological, and dietary factors on objectively measured physical activity levels in youth. The objectives of this dissertation were: 1) to describe activity levels across race/ethnic, weight status, age, and gender groups in a large nationally representative sample of youth; 2) to examine cross-sectional and longitudinal relationships between leptin and physical activity in a sample of minority female children; and 3) to assess the effects of a high sugar versus a high fiber meal on activity levels in an in-lab setting in minority adolescents.; The participants from Study 1 were drawn from a nationally representative sample of youth ages 6-19 years who participated in the 2003-2004 and 2005-2006 National Health and Nutrition Examination Surveys (NHANES). The samples were combined to allow estimates of activity levels across race/ethnicity, weight status, age, and gender groups (N=3,106). The participants from Study 2 (N=50) were Hispanic and African American females ages 8-11 years who participated in the Transitions Study, a longitudinal study on the factors contributing to the observed decline in moderate to vigorous physical activity (MVPA). The participants from Study 3 were overweight or obese (BMI ≥ 85th percentile) Hispanic males and females aged 14 to 17 years who participated in a cross-over in-lab feeding study designed to assess the acute effects of two different meals (high sugar vs. high fiber) on activity levels and insulin/glucose. All three studies use accelerometers to measure activity levels.; In Study 1, the 6- to 11-yr-olds spent more time (88 min/d) in MVPA than the 12- to 15-yr-olds (33 min/d) and 16- to 19-yr-olds (26 min/d; p = .001 for both). Females spent fewer minutes per day in MVPA than males (p = .001). Overall, obese youth spent 16 fewer minutes per day in MVPA than normal-weight youth. However, non-Hispanic white males spent three to four fewer minutes per day in vigorous physical activity than Mexican American (MA; p = .004) and non-Hispanic black (p = .001) males but had lower obesity rates. Obese 12- to 15-yr-old Mexican Americans recorded similar minutes in MVPA per day as normal-weight Mexican Americans (p = .050). There was a significant three-way age–body mass index–race/ethnicity interaction for mean minutes per day in MVPA (p = .001) such that although there were differences in physical activity in younger age groups, levels declined in the oldest age group so that youth ages 16-19 of all race/ethnic groups spent the same amount of time (between 24 and 29 minutes) in MVPA (p>.050). In Study 2, there were pubertal differences in leptin levels such that girls in Tanner stage 1 had lower levels of leptin (p= .004) than girls in Tanner stage 2. Leptin was negatively associated with MVPA, but not Tanner stage, independent of adiposity in the cross-sectional model (p = .013). MVPA declined by 12.3% (6.4 min/d) over one year. In the longitudinal model, baseline leptin predicted the decline in MVPA over one year (p = .017) independent of central adiposity and pubertal stage. In Study 3, meal condition predicted change in insulin and glucose IAUC (p< .001 for both) but not activity levels over the observation period. Insulin IAUC predicted MVPA, but did not mediate the meal condition-MVPA relationship. There were also differences in insulin and glucose IAUC at specific time points (p< .001 for all): these values were elevated in the high sugar condition at 30- and 60- minutes post-meal.; In conclusion, this dissertation lent support for individual demographic and biological basis for physical activity. In Study 1, females and older youth were the least active groups. Obese youth were generally less active, but this did not hold uniformly across race/ethnic groups. In Study 2, the inverse leptin-MVPA relationship was stable over time in minority females. Pubertal stage was not a factor in this relationship. In Study 3, meal condition caused change in insulin and glucose but not activity. The findings from Study 1 support individual demographic variations in activity. The findings from Studies 2 and 3 support a biological basis of activity. These findings inform the understanding of the individual, biological, and dietary factors that are related to the decline in physical activity in youth.

OBJECTIVELY MEASURED PHYSICAL ACTIVITY
AND RELATED FACTORS IN MINORITY YOUTH
by
Britni Ryan Belcher
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PREVENTIVE MEDICINE—HEALTH BEHAVIOR)
May 2011
Copyright 2011 Britni Ryan Belcher